This invention relates generally to microwave filters, and more particularly the invention relates to microwave notch filters employing YIG resonators.
Yttrium iron garnet (YIG) spheres serve as resonators in a host of magnetically tuned microwave devices such as YIG tuned oscillators, bandpads filters, band rejection filters, limiters, and discriminators. The YIG has high-quality (Q), inherent tuning linearity, extremely broad tuning range, good temperature stability, manageable spurious response, and small size. These characteristics make the YIG an ideal tuning element for microwave devices such as notch filters or band reject filters.
In a notch filter, a single YIG resonator typically will not supply the rejection characteristics required for modern sophisticated electronic systems. Therefore, it is standard practice to use multiple resonators to achieve usable stop band shape. A typical four-section filter may tune from 8 to 18 GHz with a 40 dB rejection bandwidth of 10 MHz, a 200 MHz maximum 3 dB bandwidth, and a passband insertion loss of 1 dB.
Electronically tunable microwave band rejection or notch filters have wide application whenever an undesired signal must be isolated, or rejected, from an electronic system. A typical application might be as a self-protecting jammer protecting an airborne broadband receiver from the aircraft's fire control radar or radars which may hop from one frequency to another frequency very quickly. As the rad-ar hops around, the notch must be tuned to the new frequency, thereby jamming it and protecting the receiver from overload, while any signal outside the notch can be received and analyzed unattenuated. Accordingly, it is often desirable to remove the notch from the band very quickly (i.e. less than 500 ns). In the case of a YIG filter, this cannot be accomplished by merely reducing the tuning coil current. The magnetic core inductance is such that time constants on the order of 50 ms are required to reduce the magnetic field strength through the YIG spheres, and consequently the resonant frequency of the notch to a level below the operating band. For this reason, another technique of disabling the notch has heretofore been in use.
The conventional solution has been to place the filter between two single-pole double-throw (SPDT) PIN diode transfer switches. The notch can be switched off (bypassed) at a very fast rate. However, there are several disadvantages to this approach including additional insertion loss from the switches, catastrophic failure if DC power is lost to the switch, and a single diode failure can render the switches and hence the receiver system inoperable.